1. Field of the Invention
This invention relates to a floppy disk drive apparatus for controlling a floppy disk drive and, more particularly, to a floppy disk drive apparatus used as an auxiliary memory of a numerical control unit.
2. Description of the Related Art
In a conventional computer system, a non-volatile external memory device generally is employed as an auxiliary memory, a magnetic disk device or the like having a large storage capacity is such a device. Recently, numerical control units for controlling NC machine tools also have come to employ floppy disks, which are inexpensive and have a large storage capacity, as auxiliary memory devices.
A floppy disk is a sheet of polyester or the like coated with a magnetic material and sealed in a jacket referred to as a diskette. The diskette has a slit for reading and writing recorded information. When the diskette is loaded in the floppy disk drive apparatus, the disk is rotated and information is read out or written in via a read/write head.
Floppy disks are of different types insofar as their storage density, size and the like are concerned, and the type used depends upon the particular application. At factory sites where machine tools are installed, 3.5 inch floppy disks, which are small in size and possess a shutter for opening and closing the recording surface to protect the same, are finding greater use.
In such devices the memory read/write head is in contact with the recording surface of the floppy disk at all times. When the disk is started rotating by an externally applied access command, input and output of data become possible. In general, disk wear time is on the order of one million passes per track; if the disk is used continuously, this is equivalent to 50 hours. In a case where the disk is used as an auxiliary memory of a numerical control unit, the disk is accessed from a main NC program by the required command. Therefore, when there is no current access with respect to a floppy disk controller, the drive motor is stopped. Unless wear of the recording surface of the floppy disk is prevented, the disk will soon reach the end of its service life and not only will the information recorded thereon be lost but this can also lead to malfunctioning of the machine tool during automatic operation.
Accordingly, the floppy disk controller in the prior art is adapted to halt the drive motor when the disk is not being accessed, thereby prolonging disk life. However, in order for a numerical control unit to execute an NC program smoothly, it is necessary for the contents of the auxiliary memory to be read in a short access time. To this end, in the prior art, the setting is such that the motor is controlled to stop at a fixed time, e.g., 30 seconds, after the floppy disk access command is completed in anticipation of the time up to the next access command. Control is such as to minimize lost time up to the moment a read-out enable state is attained when a read command is received following stopping of the motor.
With the conventional floppy disc controller described above, however, the drive motor is always made to stop after a fixed time period irrespective of the fact that the control characteristics of the disk drive motor, the speed at which data is read and the speed at which data is transferred to the numerical control unit after read-out differ from one system to another. Consequently, if the data transfer speed, for example, is low, the drive motor rotates more than necessary, thereby hastening disk wear. In other words, if the time needed for the next access command to arrive is estimated to be larger than necessary, the life of the floppy disk recording surface contacting the head for this time is shortened in proportion to the length of this time.